Add new console/http loggers (#6066)

- Add console target logging, enabled by default.
- Add http target logging, which supports an endpoint
  with basic authentication (username/password are passed
  in the endpoint url itself)
- HTTP target logging is asynchronous and some logs can be
  dropped if channel buffer (10000) is full

cmd/logger/targets.go

@@ -0,0 +1,33 @@
+/*
+ * Minio Cloud Storage, (C) 2018 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package logger
+
+// LoggingTarget is the entity that we will receive
+// a single log entry and send it to the log target
+//   e.g. send the log to a http server
+type LoggingTarget interface {
+	send(entry logEntry) error
+}
+
+// Targets is the set of enabled loggers
+var Targets = []LoggingTarget{}
+
+// AddTarget adds a new logger target to the
+// list of enabled loggers
+func AddTarget(t LoggingTarget) {
+	Targets = append(Targets, t)
+}